Biosensors, which refer to sensors utilizing a biological or biologically derived component in a transduction process, are frequently used as analytical tools in fields such as medicine, drinking water and food control. Biosensors can provide sensitive, fast, repetitive and cheap measurements for the detection, quantification and monitoring of different biological and chemical compounds.
One class of biosensor, called an ion selective field effect transistor (ISFET), is based on field effect transistor (FET) technology used in electronics, such as an enhanced mode metal oxide semiconductor field effect transistor (MOSFET). In an ISFET, a variation in the concentration of the ions of interest provides the variable gate voltage to control the conductivity of the channel. One specific type of ISFET is an enzyme field effect transistor (EnFET), which utilizes an immobilized enzyme to create a detectable species. Other types of biosensors include pH-based or ion selective electrodes. The ion or target species detected can be monitored or measured by using electrochemical, optical, calorimetric or acoustical means, for example.
A specific challenge for the use of biosensors, and specifically ISFETs and pH-based electrodes, in biological fluids, is the response of the sensor in a highly buffered environment. For example, blood plasma may contain about 40 mM of carbonate buffer. The high buffer concentration diminishes the ionic change of the analyte species that the sensor detects, therefore decreasing its sensitivity. In other situations, a conversion may occur at too high a rate for accurate detection and an increased buffer concentration would extend the dynamic range of the sensor for the target analyte or species.